Thermosetting resin compositions are used as electrically insulating materials, structural materials, adhesives and the like by casting, impregnating, laminating or molding. The recent trend is toward severer requirements on materials used in these applications, and especially the heat resistance of materials is an important requirement.
Polyimide and epoxy resins form typical classes of commonly used thermosetting resin compositions. Polyimide resins have good heat resistance, but require long-term high-temperature heating to process, that is, having a processing disadvantage. Epoxy resins which are modified to improve heat resistance are easier to process, but poor in mechanical and electrical properties at high temperatures and long-term heat degradation resistance, and high-level heat resistance.
There are known some substitutes for the polyimide and epoxy resins, including a thermosetting resin mixture comprising a polyimide and an alkenylphenol or alkenylphenol ether (Japanese Patent Application Kokai=JP-A No. 994/1977) and a thermosetting resin composition comprising a maleimide compound, a polyallylated phenol and an epoxy resin (Japanese Patent Publication 57-28416). The polyallylated phenol used in the latter composition is obtained by Claisen rearrangement of a polyallyl ether or has the structure that generates a phenolic hydroxyl group through Claisen rearrangement upon heat curing. Each substituting allyl group and the hydroxyl or ether group are at ortho positions on a common aromatic ring, and in the case of novolak type resin compositions, these groups tend to remain unreacted even after curing, leaving problems in as-cured properties and heat degradation resistance at high temperatures.